Please refer to FIG. 1, which is a partial circuit diagram showing an uninterruptible power supply (UPS) according to the prior art. The UPS 1 includes an ac input voltage AC, a switch combination composed of two diodes D1 & D2, a single-phase ac/ac converter 11, an ac filter 12 composed of a filter inductor Lo and a filter capacitor Co, and a load R.
In the single-phase ac/ac converter 11, there exist an ac inductor Li, a bus capacitor Cs and three bridge arms. The switches S1 and S2 constitute a boost bridge arm, the switches S3 and S4 constitute a common bridge arm, and the switches S5 and S6 constitute a buck bridge arm.
In the UPS 1 shown in FIG. 1, on the one hand, the ac input voltage AC provides energy to the load R when the ac input voltage AC is operating normally. On the other hand, a storage battery which is not shown provides the energy to the load R when the ac input voltage AC is operating abnormally.
As to the UPS 1, the switch loss is high since one bridge arm thereof is operated with pulse width modulation (PWM) technique. Besides, the usage of the bus capacitor Cs also interferes with the miniaturization of the UPS.